Characteristics of the shipping industry
The shipyard is very simple. Therefore, there is very little space on board. The operator must reduce the volume to make the natural gas tank as small as possible. Liquefied natural gas or LNG is adopted to realize this purpose. Natural gas is liquidized by cooling at a temperature of approximately -165°C. At this temperature, the main isolation valve must still work.
What affects valve design?
A temperature has an important influence on valve design. For example, users may need the valve for hot environments like the Middle East, or it may be suitable for cold environments like polar oceans. Both environments will affect the sealing and durability of the valve. The components of these valves include the valve body, valve cover, valve stem, valve stem seal, ball and valve seat. Because of the difference in material composition, these parts expand and contract at different temperatures.
Low temperature application options
1. Valves are used in cold environments such as oil rigs in polar waters.
2. Valves are used to manage fluids whose temperatures are far below freezing.
In the case of highly flammable gases such as natural gas or oxygen, the valve must also be operated correctly in the event of a fire.
Pressure problems
There is an accumulation of pressure in the normal operation of refrigerants due to the increased heat of the environment and subsequent steam formation. Special attention should be paid when designing the valve or piping system. This allows pressure to build up.
Temperature problems
Severe temperature changes will affect the safety of workers and factories. Because of the different material compositions and the time they are exposed to the refrigerant, each part of the
cryogenic valve expands and contracts at different rates. Another big problem when you deal with refrigerants is the increase in heat from the surrounding environment. The increase in heat is the reason why manufacturers isolate valves and pipes. In addition to the high temperature range, the valve must also deal with considerable challenges. For liquefied helium, the temperature of the liquefied gas drops to -270°C.
Function problems
Conversely, if the temperature drops to absolute zero, the valve function becomes very challenging. The
cryogenic valve connects the pipe and liquid gas to the environment. It may result in a temperature difference of up to 300°C between the pipe and the environment.
Efficiency problems
The temperature difference generates a heat flow from the warm zone to the cold zone, which will damage the normal function of the valve and also reduces efficiency of the system in extreme cases. This is of particular concern if ice forms on the warm end. However, in low temperature applications, this passive heating process is also deliberately used. This process is used to seal the valve stem. Usually, the valve stem is sealed with plastic. These materials cannot withstand low temperatures, but the high-performance metal seal of the two parts moves a lot in opposite directions. However, it is very expensive and almost impossible.
Sealing problems
There is a very simple solution to this problem. You can bring the plastic used to seal the valve stem to an area where the temperature is relatively normal. This means that the sealant of the valve stem must be kept at a certain distance from the fluid. The engine hood is like a tube. If the fluid rises from this pipe, it will warm up from the outside temperature. When the fluid reaches the stem sealant, it is mainly at ambient temperature and is gaseous. The engine hood also prevents the handle from freezing and being unable to start.
Selection of cryogenic service valves
Selecting a valve for cryogenic applications can be very complicated. Buyers must consider the conditions on the shipboard and factory. Moreover, the specific properties of cryogenic fluids require specific valve performance. The correct choice can ensure factory reliability, equipment protection and operational safety. Two main valve designs are used in the global LNG market.
Valve design conforming to the global LNG market
Three offset rotary tight isolation valves
These offsets allow the valve to be opened and closed. There is little friction in the operation. The stem torque is also used to make the valve more tight. One of the challenges of LNG storage is the medium being trapped in cavities. In these cavities, the liquid can explosively expand more than 600 times. The three offset tight isolation valve eliminates this challenge.
Single and double plates check valves
These valves are key components in liquefaction equipment, because they can prevent damage caused by flow reversal. Materials and sizes are important considerations, because
cryogenic valves are expensive. Choosing incorrect valves can be harmful.
How do engineers ensure the sealing of cryogenic valves?
When one considers the cost of first making the gas into a refrigerant, leakage is very expensive. This is also very dangerous. A big problem with the cryogenic technology is the possibility of valve seat leakage. Buyers often underestimate the radial and linear growth of stems in relation to the body. If buyers choose the right valve, they can avoid the above problems. It is recommended that cryogenic valves made from stainless steel should be used. In the process of operating liquefied gas, the stainless less material responds well to temperature gradients.
Cryogenic valves should use suitable sealing materials with sealing up to 100bar. In addition, the extended valve bonnet is a very important feature, because it determines sealing of the stem sealant.
